If you have a Fluidmaster or Korky fill valve, the replacement seal is quite easy to replace, inexpensive and readily available. (If unsure, Google "Fluidmaster 400A" and "Korky Quietfill" for pictures.) Either of these is normally an easy, suitable replacement. Detailed instructions are on the box.
If you have the type with the big plastic ball that raises as the tank fills, you'll be better off replacing it with one of those mentioned above.